1. The Field of the Invention
This invention relates to magnetically supported and rotated rotors and, more particularly, to a centrifugal pumping apparatus and method whose disk-like impeller is magnetically suspended and rotated in a contact-free manner, the rotation speed of the impeller being controlled and changed electronically by fluid pressure and impeller positioning algorithms.
2. The Relevant Technology
Historically, fluid pumps are of many and varied types and configurations, all performing essentially the same end result, namely, to provide fluid movement from one point to another. All pumps have a similar characteristic in that fluid is drawn into the pump through a vessel or pipe by a vacuum created by pump operation. In addition to the primary force of vacuum, secondary forces such as gravity, impeller inertia, or existing pipe/vessel fluid pressures also have an effect on fluid flow. Operation of the pumping mechanism creates a fluid pressure and/or fluid velocity which subsequently creates the vacuum that draws fluid into the pump through a pump inlet port. Fluid from the inlet port is transported throughout the pump by the pump mechanism which subsequently directs fluid to a pump outlet port.
Fluid pump configurations vary mostly by adaptation to function. For example, lift and force pumps utilize a reciprocating motion to displace fluid, whereas vacuum pumps create a vacuum that is used to displace fluid. Rotating axial-flow pumps utilize propeller-like blades attached to a rotating shaft to accomplish the displacement of fluid. Jet pumps utilize a steam-jet ejector which enters a narrow chamber inside the pump and crates a low-pressure area that correspondingly creates a suction that draws the fluid into the chamber from an inlet port. Although, other pump types could be specified, more specific reference will be made hereafter to fluid pumps for a sensitive fluid such as blood which are more easily adaptable to environments where size and geometry of the pump are critical.
The rotating centrifugal pump is, by nature, more tightly configured and readily adaptable to pumping of sensitive fluids. Blood flow pumps have relatively low flow rate performance characteristics compared to many ordinary industrial applications yet have significant pressure rise requirements. Centrifugal pumps are well suited to such applications rather than axial flow pumps or other designs. This leads to the use of a centrifugal pump design for the preferred embodiment of this invention. The pump includes several ribs or vanes mounted to an impeller whose rotational force impels fluid toward the outside of the rotor by centrifugal force. Centrifugal pumps traditionally possess a shaft-mounted impeller immersed in the fluid, where the shaft extends through a seal and bearing apparatus to a drive mechanism. Revolving vanes of the impeller create a partial vacuum near the center of the axis of rotation which correspondingly draws in fluid through the intake opening of the pump. A smooth pump volute is located in the pump stationary component to assure the smooth flow of pumped fluid from the exit of the impeller to the pump exit passage. The volute accumulates the pump flow as it exits the pump impeller and performs the function of increasing the fluid pressure (head) by converting fluid kinetic energy (velocity) to potential energy (pressure or head). Although centrifugal pumps do not require valves for movement of fluid, pump geometry must be such that fluid drawn in through the input opening will continue through the pump mechanism and on to the outlet port without significant internal fluid leakage or inefficiencies.
These prior art pumps are known to have problems. For example, it is well documented that shaft seals as configured in conventional centrifugal pumps are notoriously susceptible to wear, failure, and even attack by certain fluids, thus resulting in leakage problems. It is also well known that pumps for some fluids require more careful design consideration and require specific pumping techniques in order to avoid fluid damage, contamination, and other undesirable conditions. For example, fluids such as corrosive fluids (acids or caustics) or sensitive fluids such as blood, require special consideration such that seals do not leak and thereby lose integrity of the fluid. Pumping of sensitive fluids, such as blood, by continuous flow pumps requires highly reliable and non-damaging bearings to support the rotating impeller. Prior art pumps have very significant problems with bearings needed to support the impeller as it rotates. Ball and other rolling element bearings can only be employed if isolated from the sensitive fluid (blood) by shaft seals and lubricated with non-body fluids. In this situation, all of the sealing problems indicated above apply. If the conventional ball or other rolling element hearings employ the sensitive fluid as a lubricant, the sensitive fluid living properties, such as red blood cells in blood, are destroyed in a short period of time due to being ground between the rolling components in the bearings. Thrust and radial fluid film bearings, lubricated with the sensitive fluid, have been employed in some prior art pumps. These have been subject to poor performance and/or many failures due to seizure of the rotating component in the stationary component, production of thrombosis (clotting), damage to the sensitive fluid due to hemolysis (high shear), and other problems.
Fluid film bearings also do not provide any information on the instantaneous pump pressures and flow rates that can be employed for speed control of the motor to match physiological needs to future pump performance. Conventional ball bearings and fluid film thrust and radial bearings do not have the long term reliability required for pumps in which fluid stasis and high fluid shear stress must be avoided, such as blood pumps. Furthermore, ball bearings have a limited life when employed in the pumping of sensitive fluids and often must be lubricated by an external lubricating fluid which requires seals to contain the lubricating fluid. Transport and containment of lubricating fluid for bearings increases the overall size of the pump housing as well as increasing complexity of operation due to extra vessels and mechanisms used to deliver and cool lubricating fluid, thereby making pump apparatus non-implantable infused to replace natural heart functions. Therefore, the relatively short life of fluid pumps with shafts and conventional bearings makes them unsuitable for implanting in body cavities for the long term replacement of natural heart functions.
Furthermore, pumping of blood involves specific known hazards typically associated with shaft seals for impeller-type blood pumps due to pockets of fluid being susceptible to stagnation and excessive heat. Further still, pumping sensitive fluids, such as blood, requires careful consideration of geometry of impeller vanes and pump housing. Excessive mechanical working and heating of blood causes blood components to breakdown by hemolysis and protein denaturization, which leads to blood coagulation and thrombosis.
Avoidance of blood damaging effects of pump operation is best accomplished by natural heart function. The natural heart has two basic functions, each side performing a different pumping function. The right side of the natural heart receives blood from the body and pumps it to the lungs, whereas the left side of the natural heart collects blood from the lungs and pumps it to the body. The beating of the natural heart, in combination with heart valves, provides blood pumping action in a pulsatile, remarkably smooth and flowing manner. Blood flow (cardiac output) of the natural heart is primarily regulated by venous return, otherwise known as pump preload. However, due to diseases or accident, natural heart functions can be partially or totally lost. Mechanical apparatus developed to replace natural heart functions historically ranged in size from extremely large in the earliest heart-lung or pump oxygenator apparatus to more recent apparatus whose size and function more closely resembles that of the natural heart.
In addition to total heart replacement, development of other mechanical apparatus focuses on replacement of a portion of the function of the natural heart, such as a ventricular assist device that aids a failing left ventricle weakened by disease or other damage. A primary consideration for natural heart function replacement, whether partial or total, is that blood must be pumped throughout the entire apparatus in a gentle, low thermal, and non-destructive manner. For example, if a pump impeller supported by mechanical bearings comes in contact with blood, relative movement between parts of the bearings results in excessive mechanical working of the blood which causes blood cells to rupture, resulting in hemolysis. Another mechanical effect that can injure blood is formation of regions within the pump where blood is semi-stagnant or where blood will eddy without sufficient blood exchange, thereby creating the equivalent to blood stagnation. The result of blood stagnation often is coagulation of the blood (thrombosis), which correspondingly causes blood to cease to flow at all. Yet another effect that can injure blood is excessive heating due to friction of a sidewall of the pump or other pumping mechanisms as blood passes through the pump. Specifically, side wall friction caused by abrupt angular changes of internal pump geometry requires blood to follow harsh changes of direction and thereby creates excessive mechanical working of blood which causes blood cell rupture or activation of blood platelets and corresponding hemolysis and thrombosis. Yet another effect that can injure blood is caused by inefficient pump operation whereby a large part of the energy supplied to the pump appears as heat discharged into the blood which damages blood by overheating and coagulation. Notably, because blood albumen begins to denature at 42 degrees Centigrade, inefficiencies in pump operation which result in overheating of the blood will cause a very serious and life threatening condition.
The before mentioned conditions of stagnation, harsh pump geometry, turbulence and/or heating will activate blood platelets and/or damage oxygen-carrying red blood cells. Damage to blood starts a chain reaction that forms a thrombus with potential to block blood vessels, starving the tissues it nourishes, and leading to a serious, life threatening condition. Numerous attempts to avoid the foregoing problems associated with pumping blood have been made using flexible diaphragms and collapsible tubing in roller pumps However, the continual flexing of the diaphragm and/or tubing material is known to change the blood-contacting properties of the material resulting in material fatigue, dislodged fragments of the internal wall of the flexible material, and emboli passed into the bloodstream by the fragments.
In addition to the above mentioned conditional requirements for pumping blood, the rate of impeller rotation has a significant effect on stability and structure of sensitive vessels. Impeller rotational operation that is not regulated by pump preload pressure will cause atrial suction in sensitive vessels just prior to the pump inlet port, wherein blood vessels collapse when impeller rotation exceeds blood vessel wall rigidity. Prior art pumping apparatus has not provided adequate integration of controls to insure that rapid adjustments to impeller rotational speed does not have a negative effect.
Kletschka ""005 (U.S. Pat. No. 5,055,005) discloses a fluid pump levitated by opposing fluid. Stabilization of impeller by opposing fluid alone is not sufficient to maintain impeller in precise position within pump housing, as well as high pressure fluid jets subject blood to the before mentioned blood coagulation caused by mechanical working of blood.
Kletschka ""877 (U.S. Pat. No. 5,195,877) discloses a fluid pump with a magnetically levitated impeller utilizing a rigidly mounted shaft surrounded by a magnetically levitated rotor which serves as an impeller for fluid. The shaft of this invention introduces a requirement for a hydraulic bearing and seal at the juncture of the shaft and the rotating impeller which subjects blood, or other sensitive fluids, to thermal and stagnation conditions at the region of the bearing.
For more than 25 years, those skilled in the art have studied pumps that are used as total artificial hearts and experimentally implanted in animals. These studies have provided useful feedback of the relative effectiveness of blood pumping apparatus. These pumps can be categorized a, producing pulsatile or non-pulsatile flows. The pumps producing pulsatile fluid motion (positive displacement pumps) more closely resemble fluid motion as provided by the natural heart. Information to date has not yet determined if pulsatile fluid movement is needed to provide a necessary physiological benefit, or if the pulsatile fluid motion is primarily due to the non-rotary nature of heart muscle. Most pulsatile pumps universally require valves (mechanical or tissue) with inherent mechanical problems and limitations. valve systems are not required in prior art non-pulsatile pumps, the non-pulsatile pumps require rotating shafts passing through various bearings and seals. These shafts create inherent problems of blood stagnation, contamination and undesirable thermal conditions, thereby making long term use of the pumps as a replacement for natural heart function unfeasible. Most early prior art rotating non-pulsatile systems were installed outside of the body for short-term cardiac assistance and experienced a moderate amount of success.
One blood pumping apparatus is the total artificial heart. The total artificial heart has been used in five patients as a permanent replacement for pathological, irreparable ventricles; and in 300 patients as a temporary bridge to cardiac transplantation. The longest support on the total artificial heart has been 795 days. Other blood pumping apparatus, e.g., ventricular assist devices, have been used in patients unweanable from cardiopulmonary bypass during cardiac surgery or those whose one ventricle only has failed. The most common mechanical replacement of natural heart function is a temporary bridge to cardiac transplantation by a ventricular assist device with over 1250 patients receiving such temporary ventricular assist apparatus.
Historically, blood pumping apparatus have presented many problems. For example, the pumping mechanism of reciprocating (diaphragm) total artificial hearts has been energized with gases (pneumatic systems), electricity (motors, solenoids, etc.), and skeletal muscles. The energy sources and associated convertor systems possess additional components that increase complexity of the total system and thereby contribute to overall unreliability. Also, the size of prior art systems for total artificial hearts is very restrictive to patient mobility and not conducive to quality of life of the recipient. Another constraining factor not fully met by prior art apparatus is that the excessive size and complexity of energy conversion systems, as well as overall pump design exceeds the available anatomical space. Furthermore, most of these prior art reciprocating systems exhibit excessively high (i) noise characteristics, (ii) vibration, and (iii) recoil (thrust) levels.
Many of the problems of the prior art rotating pumps have been addressed by those skilled in the art through pump adaptation with capability to meet the above mentioned requirements for pumping sensitive fluids (such as blood). These pump adaptations can be accomplished bag support of the impeller through electromagnets located on the impeller and the housing such that the impeller can be rotated without shafts, seals or lubricating systems. Permanent magnets without some form of additional support cannot entirely suspend an object, such as an impeller, but require additional adjustable support or force in some axis to achieve stabilized suspension. This is based on Earnshaw""s theorem which indicates that suspension systems comprised solely of permanent magnets will not be stable. However, actively controlled electromagnets can be used to stabilize and support an object with respect to all degrees of freedom of movement. Additionally, one electromagnet with a feedback position sensor can provide stable suspension of an object (or impeller in the case of the centrifugal fluid pump). The only expenditure of energy in hybrid magnetically supported impellers is electromagnetic energy utilized for stabilizing and rotating the impeller. Permanent magnets and one electromagnet for impeller suspension and rotation create a stable and efficient pump operation.
Within the past decade, prior art patents have disclosed magnetically suspended and rotated rotors which have exhibited a limited degree of success. These prior art configurations utilize partial magnetic suspension to reduce hazards to blood. Although magnetically suspended prior art devices successfully reduce some of the friction hazard of the rotary shaft, the prior art devices are still impractical for implantation in total heart replacement due to size, complexity, and less than optimal impeller positioning, position sensing, and speed control. The excessive size and difficulty in maintaining precise impeller positioning and speed of these prior art inventions is due mostly to geometric configuration of the impeller, which is cylindrical, spherical, or otherwise mostly three dimensional in nature.
In view of the foregoing, it would be a significant advancement in the art to provide improvements in magnetically suspended and rotated centrifugal pumping apparatus to thereby allow for reduced size and increased accuracy in impeller positioning and speed controls. It would also be an advancement in the art to provide a centrifugal pumping apparatus that would be free of shafts, rolling element or fluid film bearings, mechanical seals, or physical proximity sensors, thereby allowing for a fully integrated pump design without mechanical contact, wear, failure due to seizing up of fluid bearings, and generation of thrombosis or shear damage. An even further advancement in the art would be to provide a centrifugal pumping apparatus with geometry of impeller and pump housing such as would provide efficient and low-turbulence transport of fluid throughout pump mechanisms including the pump output port. Further still, it would be an advancement in the art to provide a versatile centrifugal pumping apparatus that could operate in either pulsatile or non-pulsatile mode.
It is a primary object of the present invention to provide improvements in rotating centrifugal fluid pumps for sensitive fluids.
It is another object of the present invention to provide improvements in fluid pumps using a combination of permanent magnets, efficient non-contact electromagnetic bearings, and an efficient motor.
It is also an object of the present invention to provide a centrifugal pumping apparatus of relatively compact size to enable anatomical implantation.
It is a further object of the present invention to provide a centrifugal pumping apparatus and method to provide a long product life and which requires minimal maintenance.
It is an additional object of the present invention to provide improvements in centrifugal fluid pumps which are used for partial or total heart function replacement.
It is still another object of the present invention to provide a centrifugal pumping apparatus and method whose pump design geometry provides efficient and low-turbulence transport and output of sensitive fluid throughout the pump, including low-turbulence output just beyond the outlet port.
It is yet another object of the present invention to provide a centrifugal pumping apparatus and method whereby fluid pressure and output fluid volume are controlled and changed electronically via specific fluid pressure and positioning algorithms.
It is another object of the present invention to provide a centrifugal pumping apparatus and method that is capable of operation in either pulsatile or non-pulsatile mode.
It is yet another object of the present invention to provide a centrifugal pumping apparatus and method that is adaptable as either a ventricular assist device or paired to provide a total heart replacement.
The above objects and others not specifically recited are realized through an apparatus and method for a centrifugal fluid pump for pumping sensitive biological fluids, which includes (i) an integral impeller and rotor which is entirely supported by an integral combination of permanent magnets and electromagnetic bearings and rotated by an integral motor, (ii) a pump housing and arcuate passages for fluid flow and containment, (iii) a brushless driving motor embedded and integral with the pump housing, (iv) a power supply, and (v) specific electronic sensing of impeller position, velocity or acceleration using a self-sensing method and physiological control algorithm for motor speed and pump performance based upon input from the electromagnetic bearing currents and motor back emfxe2x80x94all fitly joined together to provide efficient, durable and low maintenance pump operation. A specially designed impeller and pump housing provide the mechanism for transport and delivery of fluid through the pump to a pump output port with reduced fluid turbulence.
These and other objects and features of the present invention will become readily apparent from the following description in which preferred and other embodiments of the invention have been set forth in conjunction with the accompanying drawings and appended claims.
The underlying rationale for a rotating centrifugal pump with an impeller, fully supported by a combination of permanent magnets and electromagnetic bearings and rotated by an electric motor, is to prevent damage to blood or other sensitive fluid due to conditions of (1) excessive heat, (2) stagnation, (3) coagulation (thrombosis), or (4) high shear of fluid or blood components (hemolysis) due to fluid instability caused by turbulence or mechanical working of fluid due to harsh pump mechanism or geometry. Furthermore, the apparatus size of this invention is capable of fitting into available anatomical space if used for total natural heart replacement or ventricular assistance.
To be suitable as a blood pump, the pump must be able to adequately meet physiological perfusion needs of a ventricular or biventricular assist device for total heart replacement. As a total heart replacement device, the pump must be of sufficiently small size and mass to be implantable within available anatomical space and not cause any negative effects on surrounding organs due to excessive apparatus weight. Furthermore, the disc-like shape of the impeller of this invention significantly reduces size and complexity of the pumping apparatus. The pumping apparatus of the invention can be used singularly as a ventricular assist device that assists or replaces partial heart function or a pair of devices can be combined to form a total mechanical heart replacement. The combined size of two devices in a total mechanical heart replacement is approximately the size of a natural heart, thereby enabling implantation within existing anatomical space.
The impeller of this invention is entirely suspended and enclosed within its pump housing, thereby providing contact-free operation between pump impeller and any other portion of the pump. The pump impeller is magnetically suspended with a combination of permanent magnets and electromagnetic bearings. The permanent magnets are configured in reverse polarity which provide positive radial stiffness while being employed in the radial gaps inherent in disk-shaped impellers unlike the repulsive permanent magnet rings cited in the prior art patents which can only be employed in axial gap configurations. This reverse polarity permanent magnet configuration is required for a disk-shaped impeller geometry. It is enclosed within its pump housing, thereby providing contact-free operation between pump impeller and any other portion of the pump. The pump impeller is suspended by a combination of permanent and electromagnetic forces. An electric motor rotates the pump impeller to perform the pumping function of fluid. The notable absence of shafts, ball bearings, shaft seals or other sources of contamination make possible significantly extended product life of the pumping apparatus of this invention, thereby enabling long term natural heart replacement.
The pump impeller rotates about an axis and the term xe2x80x9caxial directionxe2x80x9d is employed here to denote the direction parallel to the axis of rotation of the pump impeller. The term xe2x80x9cradial directionxe2x80x9d is used here to denote directions perpendicular to the axial direction. The invention consists of permanent and electromagnetic bearings, comprising magnetic and other materials, activated by electrical currents in coils wound around the bearing magnetic components, which develop axial forces and provide adjustments to impeller positioning relative to pump housing. A multiplicity of magnetic bearings, in a suitable configuration arranged around the impeller, is required to center the impeller during operation of the pump and to avoid contact between the rotating and stationary components. Six impeller degrees of freedom: three translations and three rotations, must be controlled. This non-contacting operation allows the bearings to operate without wear or friction losses.
A feedback electronic controller is provided in the suspension system to automatically adjust the activating (thrust) bearing coil currents which, in turn, adjust the control forces exerted by the magnetic bearings on the rotating impeller in response to the applied forces. Such electronic controller is continuously provided with an electronic signal which is related to the position or velocity or acceleration or a combination of position, velocity and acceleration, of the rotating impeller in the available clearance space inside the pump frame during operation. Switching or direct current power amplifiers and power supplies necessary to operate the electromagnetic actuators in the magnetic bearings are provided in the invention.
Impeller position and rotational speed of this invention are controlled by specific algorithms which sense fluid pressure and the axial location of pump impeller within pump housing, correspondingly making adjustments to rotational speed and/or impeller position to provide a fully integrated system of physiological control. Impeller rotational speed is adjusted to correspond to fluid pressure at pump preload pressure (inlet pressure) and/or exit pressure to match bodily needs for increased or decreased pump flow rate or pressure rise. This also avoids excessive rpm and thus suction thereby avoiding excessive pressure.
The geometric design of the pumping apparatus of this invention provides fluid movement throughout the entire pump mechanism in a smooth, non-turbulent, and low thermal manner. Impeller rotation causes fluid to move centrifugally by specially curved impeller vanes which emanate from the epicenter of the disc-like impeller and extend toward the outside of the impeller, and simultaneously create a partial vacuum at the region near the impeller""s axis of rotation that draws additional fluid into the inlet port. Blood, or other sensitive fluid, does not stagnate at any location within the pumping apparatus due to return fluid flow along the side of the impeller which returns fluid to the impeller epicenter without flow interference from stagnation pockets, bearings or seals. Importantly, the geometry of the pump housing, the impeller vanes, the outlet port, and all other aspects of the pumping apparatus of this invention are such that sensitive fluids are protected from damage otherwise caused by stagnation, excessive heat, turbulence, and excessive mechanical working of the fluid.
The fluid is transported throughout the entire pumping apparatus without harsh angular redirection to flow. The configuration of pump housing is designed with a spiral volute curve such that the same curve slope throughout the pump housing enables fluid to be transported within the pump housing with no net abrupt angular change of direction, nor corresponding net increase in thermal friction and energy loss due to friction from the pump side wall.
Another important feature of the pumping apparatus of this invention is the capability of operation in either pulsatile or non-pulsatile mode. Cyclic variance of impeller rotational speed will cause the pump to operate in a pulsatile mode, which more closely resembles pumping action by the natural heart, whereas uniform impeller rotational speed operates the pump in non-pulsatile mode. Operational mode change from pulsatile to non-pulsatile or vice versa is accomplished through changes to the pump operation settings, thereby avoiding trauma associated with replacing the total pumping apparatus when a change from either pulsatile or non-pulsatile is determined to be the preferred operation mode.
One aspect of this invention, unlike prior art devices, is that means or structure are provided in the magnetic suspension system to generate the electronic feedback signal related to the position, velocity or acceleration of the rotating impeller either via a physical medium such as an eddy current, induction, optical, capacitance or other approach, or via a self-sensing electronic signal obtained from the current or voltage wave form, or a combination of the current and voltage wave form provided to the activating coils in the magnetic bearings. In the case of a physical sensor device placed in the pump frame near the clearance gap between the frame and the rotating impeller, the gap between the frame and the rotating impeller, the electronic position, velocity, or acceleration signal, is obtained from signal conditioning electronics. Wiring is provided for input of the signal into the electronic controller for the magnetic bearings. In the preferred embodiment, a self-sensing signal is used and the signal conditioning is provided for determining the position, velocity, or acceleration of the rotating impeller without a physical device, which allows for a minimum number of wires required in the wiring pathways between the electromagnetic actuators and the electronic controllers.
The electromagnetic bearings and their control electronics possess a physical sensor or self sensing signal such that forces (velocity, or acceleration) attempting to displace the impeller are immediately sensed and the current delivered to the coils is altered, thus avoiding impeller displacement resultant from those forces.
The flow (cardiac output) of the natural heart is primarily regulated by the venous return (preload). Another very important feature of the invention, named the physiologic controller, provides a signal which is used to determine changes in the preload or filling pressure to the pump. The controller sends a signal from monitoring changes in current flows in the thrust bearing. This information is employed to control the rotational speed of the impeller, to regulate pump suction pressure, and to regulate the needed pump outputs. This unique feature of the magnetically suspended pump allows for sensing of the inflow pressure (preload) and thus the flow (cardiac output) consistent with the physiologic needs of the recipient as a ventrical assistance device (VAD). When two pumps are used as a total artificial heart (TAH) the rotational speed of each pump will be regulated independently and each pump will be sensitive to the preload thus providing changes in flow and balance consistent with the changing physiologic needs of the recipient. This feature allows the pumps to be used without the need of the complexities associated with volume displacement chambers required with pulsatile pumps.
The invention provides for a motor to impart the necessary torque and rotation to the rotor. This is a three phase brushless DC motor controlled by using back EMF. The motor is in the shape of a disc located in the base of the housing frame and near the center of rotation of the impeller. Commutating the motor with back EMF allows effective start-up and precise control of the speed of rotation. Changes in the rotation speed are predicated on the preload as described above in the form of a physiologic controller.